Goldenrod Ball Galls -- © Dave Spier

Pair of ball galls on Canada Goldenrod stem at Montezuma Audubon Center - © Dave Spier

In August and early September, the field had been bright yellow with goldenrod flowers. Now it’s turning brown, although this too can be attractive when the plants are frosted and backlit by the rising sun. In November, most of the goldenrod leaves will shrivel and drop, exposing abnormal swellings on many of the plant stems. The common ones found on Canada Goldenrod are ball-shaped, resembling miniature basketballs about an inch or more in diameter. Not surprisingly they are usually called "goldenrod ball galls."

To fully understand galls we need to cut one open and look inside. Carefully pry and twist while cutting into the pithy core and try to pop the gall open. At the very center is a small cavity with the even smaller white larva of the peacock fly, Eurosta solidaginis, a relative of the fruit fly. Originally injected into the stem as an egg, it hatched and began feeding. Chemicals in its saliva stimulate the host plant to produce a pithy bulge in an effort to heal the "wound" caused by the parasitic larva. This actually benefits the insect by providing endless food and later an insulating shelter with a hard, weatherproof shell.

The gall, however, is not impermeable. Downy Woodpeckers and Black-capped Chickadees have learned that galls contain a nutritious morsel, and these birds will peck and chisel until they get it. You can recognize their work by the rough, funnel-shaped excavations they leave behind.

Gall fly larvae that escape this fate will chew a tunnel almost to the surface and there they will pupate until they emerge. Winged adults lack chewing mouth parts and can only pop the lid. They do this by inflating a balloon-like bladder (called the ptilinum) between the eyes. You can recognize their work by the small, sharp-edged hole that appears to have been drilled into the gall. The only function of the adult stage is to find a mate and lay more eggs in more goldenrod stems.

Goldenrods are prone to several other kinds of galls. An irregular-shaped growth called the "knotty goldenrod gall" is caused by the gall midge Lasioptera solidaginis. It’s somewhat globular, but not pith-filled. Less obvious is a long, spindle-shaped bulge called the "elliptical goldenrod gall" caused by the caterpillar of a moth.

A third type is very different in structure and occurs at the top of the stem. The "goldenrod bunch gall" is actually a globular head of deformed leaves about two inches in diameter. It is caused by a gall midge, a type of fly related to the mosquito. Its egg-laying apparatus, called an ovipositor isn’t strong enough to pierce the tough stem, so it contends with laying its egg on the tip of the plant.

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